Subsequent Events - Additional Information (Detail) - Nestlé Health Science [Member] - USD ($) $ in Thousands	Nov. 01, 2018	Jan. 09, 2016
Subsequent Event [Member]
Subsequent Event [Line Items]
Upfront collaboration milestone payments receivable	$ 40,000
Phase 2 [Member]
Subsequent Event [Line Items]
Upfront collaboration milestone payments receivable		$ 20,000
Phase 3 [Member]
Subsequent Event [Line Items]
Upfront collaboration milestone payments receivable		$ 20,000